Today, continuous digesters for instance comprise substantially cylindrical pressure vessels arranged in an upright position. Pressure vessels of continuous pulp digesters are remarkably high, and their diameter can be several meters. For instance, the diameter of the bottom part of a pressure vessel may be 4 m to 15 m, all depending upon production capacity of the digester.
The diameter of such a pressure vessel is usually smaller at the top end of the digester than at its bottom end. However, the diameter of the digester is typically arranged to increase at certain positions in step-outs during the cooking process by means of one or more conical transition sections. The exact positions are defined by process-specific technical grounds and where a change of treatment liquor needs to be made, using withdrawal or extraction screens at these positions. Screens are usually mounted below the above-mentioned step-outs.
Due to the considerable size of a pressure vessel, deviations from theoretical shapes of a cylinder produced during the manufacture of the pressure vessel, are significant. These deviations from the intended circular cross-section cause many problems. One problem arises, for instance, when the inner surface of the pressure vessel is provided with the screens required. Besides these deviations of the cylindrical shape from manufacturing are the inside of the digester wall exposed to both erosion and formation of deposits during operation of the digester.
Conventionally, this type of screens are mounted so that the step-out which increases the diameter of the pressure vessel is provided with a screen surface below the step-out such that the internal diameter of the vessel above the step-out is the same or smaller than the internal diameter at the screen area, and below a screen surface is the pulp column allowed to expand in a step-out from the screen area and towards the inside of the pressure vessel. This step-out below a screen surface is made in order to allow the chip column to expand as it may have been compacted over the screen surface during treatment liquor withdrawal. If more than one screen row is arranged in connection with such a step-out is also a smaller step-out arranged between a first upper screen row and a second screen row below, allowing the pulp column to expand and thus improve withdrawal capacity in the second screen row.
Forces directed to the surface of screens are usually arranged to be transmitted to the cover of the pressure vessel by means of two kinds of support system.
The first kind of support system is used preferably in sturdy self supporting screens where support only is obtained by a frame bed surrounding a rectangular screen area. This kind of support system has preferably been used for profile bar screens. By such support system could the withdrawal space behind the screens collecting the withdrawn treatment liquor avoid any obstructions which may accumulate deposits, and the withdrawal flow of treatment liquor could be maintained at full capacity over time.
The second kind of support system, preferably used for slotted screen plates, are support rods, i.e. bars of specific length fixed to the screen plate, arranged between the screen plate and the inside wall of the pressure vessel. This second kind of support system use a multitude of support rods each having to be adjusted to the specific curvature of the vessel wall in order to obtain a support. The asymmetry of the inside wall of the digester's pressure vessel tend to cause problems. Due to the deviations of the cylindrical shape of the pressure vessel, a considerable number of support rods are not supported onto the vessel wall, but at their one end, the support rods are hanging freely in the liquor collection chamber formed behind the screen, which collection chamber is formed between the screen and the interior wall of the pressure vessel. The object of transmitting forces directed to the screen construction to the inner wall of the digester vessel is thus not always achieved by using support rods. Because of the disadvantageous force stress the support rods bend and the screen twists, even breaks. In order to solve this problem could these support rods be made adjustable in order to avoid this problem, which requires a time consuming procedure for adjusting each individual support rod. Installation and replacement of such screen plates with adjustable support rods will be a rather time consuming process.
Examples of above described solutions could be found in prior art patents.
In US2003/0095901 is shown a support system for screen plate wherein adjustable support pins are used.
In US 2005/0284594 is shown a support system for sturdy self supporting profiled bar screens using circular profile bar rods. The profile bar screen is supported by a frame bed surrounding the screen, and horizontal support arches having integrated support shoulders are located at a distance from the digester wall in order to allow a certain ability to move. Similar profiled bar screens is also shown in U.S. Pat. No. 6,889,851 having adjustable support pins.
In U.S. Pat. No. 5,827,401 is shown yet a support system for sturdy self supporting profiled bar screens using T-shaped profile bar rods, but applied in circular screens. In this design is the force on the screen profile bars transmitted first to a support arch behind rods, and then further to a frame bed located around the circular screen.
In WO 95/16817 is yet another support system for sturdy self supporting profiled bar screens using T-shaped profile bar rods, but here using horizontally oriented profile bar rods.
In WO2013105888 is disclosed another self-supporting system for digester screens where the support arch has integrated support shoulders finding support in the outer digester wall at regular intervals and having a slot trough the support shoulders enabling a local deformation such that the support shoulders may find direct contact with the digester shell.
Profile bar screens are most often preferred as the total withdrawal area (I.e. slot area) are larger than for slotted screen plates, hence the withdrawal capacity per surface area could be higher. However, these profile bar screens are most often made with a sturdy self supporting design where all the force is transmitted to digester wall by a supporting frame bed.
One more advantage with profile bar screens is that if these have vertical profile bars with vertical slots in-between, could the slots be subjected to a continuous rubbing action from the descending pulp column keeping the slots free from any obstructions, and lower ends of these profile bars could have an unobstructed slot ending allowing any chip fragments caught in a slot from escaping out and away from the slots by the pushing action from the pulp column.
In digesters having screen areas equipped with screen plates and a multitude of support rods between screen plate and digester vessel wall, are no sturdy frame bed structure at hand which may be used to install sturdy self supporting profiled bar screens. If a profiled screen bar is to be installed in such digester it must have the same support structure with a multitude of support rods, and installation and adaption of each individual support rod becomes time consuming and thus costly. Moreover, flush out of sedimentations behind such screens with support rods during annual shut down for service work becomes difficult as the high pressure jet lances used must penetrat between all rods and direct the flushing action towards each pin. Otherwise must a new frame bed structure be welded into place which is a time consuming process, besides adding additional costs for the frame bed. This requires the mill operator to shut down the digester during annual overhaul work much longer than necessary, if new screens are to be installed.